How to Fix Old Hardwood Floors With Gaps and Cupping

The probability of warping increases as a floor ages.

Wood expands and contracts with seasonal differences in humidity, and the older a floor gets, the greater the chance that these movements will cause cupping and gaps. Cupping occurs when the boards swell and push against each other, raising their edges; gapping occurs when they shrink. If the problem is confined to one or two boards, you can replace them, but it's more common for the problem to be widespread on an old floor. The solution is to fill and re-sand, but you should only do this if there's no danger of over-sanding.

1

Examine the cross-section of the floor in a doorway to determine its sanding history. New floorboards are usually 3/4 inches thick, and to level a cupped floor, you may have to remove as much as 1/16 inch or more of the wood. Boards that are less than 5/8 inches thick should probably be replaced.

2

Wash the floor with a high-quality, wax-cutting hardwood floor cleaner. Any residual wax on the floor can gum up the sandpaper and make sanding more difficult. After cleaning, go over the floor with a hammer and nail set and pound down any protruding nails.

3

Transfer the contents of a 1-quart container of latex floor filler into a 2-gallon or larger bucket and thin it with water until it is pourable, but still stiff. Use a filler that matches the color of the unstained wood.

4

Spread the filler over the entire floor with a rubber-faced grouting float, using the float to work it into all the gaps between boards. Scrape off the excess with the float, but it doesn't matter if some dries on the surface. It will come off with the sander.

5

Sand the floor with a drum sander and 20- or 36-grit sandpaper, depending on the condition of the boards and their hardness. Sand the floor diagonally across the grain. This is the most efficient way to level it. When you're done, sand again along the grain to erase the cross-grain scratches.

6

Vacuum the floor, switch to 80-grit sandpaper and sand the floor again along the grain. If the floorboards are maple or some similarly hard wood, you may have to do an intermediate pass with 50-grit paper. Do a third pass with 100-grit sandpaper.

7

Use a flooring edger, which is a powerful disk sander on wheels, to sand along the edges of the floor. Remove finish from corners and under cabinets with a pull scraper.

8

Vacuum the floor after the final sanding, wipe it down with a damp cloth and apply stain. The stain will help hide the filler. Let the stain dry overnight.

9

Finish with three or four coats of clear finish. Apply each coat with a finish applicator and let tit dry. Scuff each coat except the last with a floor buffer and a 120-grit sanding screen. Polish the final coat with paste wax or floor polishing compound.

Things You Will Need

Hardwood floor cleaner

Hammer

Nail set

Latex floor filler

2-gallon bucket

Rubber-faced grouting float

Drum Sander

Sandpaper, various grits

Vacuum

Floor edger

Pull scraper

Damp cloth

Stain

Clear finish

Floor finish applicator

Floor buffer

120-grit sanding screen

Paste wax or floor polishing compound

Tip

If your examination of the floor reveals that the boards are engineered with a veneer, then you'll probably have to replace the floor. Sanding engineered boards enough to correct cupping will probably wear off the veneer.

Warning

Floor sanding is dusty work. Wear a dust mask and cover the doors and windows with plastic. It's also important to cover air intake vents to prevent the dust from getting sucked into the central air system.

About the Author

A love of fundamental mysteries led Chris Deziel to obtain a bachelor's degree in physics and a master's degree in humanities. A prolific carpenter, home renovator and furniture restorer, Deziel has been active in the building and home design trades since 1975. As a landscape builder, he helped establish two gardening companies.